Distortions of C604− studied by infrared spectroscopy

The Jahn‐Teller effect plays a crucial role in the explanation of the insulating character of A4C60 (A = K, Rb, Cs). To detect possible phase transitions arising from the interplay between the molecular Jahn‐Teller distortion and the distorting potential field of the counterions, we measured the mid‐IR spectra of A4C60 compounds in the temperature range 90 – 300 K and found significant spectral changes with temperature in all three compounds. We also compare these spectra to that of Na4C60 in its room‐temperature polymeric phase, where the distortion is more pronounced and evident from the structure.